Effect of the Surface Properties of γ-Al2O3 on the Conversion of Methanol into Dimethyl Ether According to in situ IR-Spectroscopic Data

Abstract

The samples of alumina of different origin exhibit different activity in the reaction of methanol dehydration to dimethyl ether (DME). To elucidate reasons for differences in their activity, a comparative study of the structural (XRD analysis, specific surface area, and porosity) and surface (Bronsted and Lewis acidity and hydroxyl coverage) properties of alumina samples was carried out. The presence of formates, aldehyde complexes, and bridging and terminal methoxy groups in different ratios on the surface of aluminum oxide samples under the reaction conditions of methanol conversion was detected using in situ IR spectroscopy. The reactivity of the detected surface compounds was compared for all of the samples. It was found that the activity of the samples in the formation of DME correlated with the rate of conversion of bridging methoxy groups. In turn, the rate of conversion of the bridging methoxy groups was determined by the acidity of bridging hydroxyl groups. An increase in acidity led to an increase in the bond strength of the methoxy group with the surface and a decrease in the rate of its conversion into DME. Linear methoxy groups underwent oxidative conversion into formate and aldehyde surface complexes. The nature of the oxidizing ability of an alumina surface is discussed.